1. Field of the Invention
The invention relates to a process for establishing a connection between steam generator heating surfaces made of an austenitic material and a vessel employed as a collector or distributor including a basic body of ferritic or martensitic material. The invention also relates to a vessel conceived as a collector or distributor including pipe nipples for connecting to a heating surface of a steam generator.
2. Description of the Invention
In modern steam generators operating at high pressures and high temperatures some of the heating surfaces of the superheater and intermediate superheater are made of austenite materials. The austenite materials have an adequate strength, corrosion and oxidation resistance at such high pressures and temperatures for the purpose of employment with such heating surfaces. On the other hand, outside of the heated space, that is to say, outside of the combustion and radiation space martensitic or ferritic materials are employed. This applies in particular also to the collectors and distributors, which are positioned in the unheated region and to which the connecting ducts are connected which are likewise produced from austenitic material.
However, the austenitic materials have a higher coefficient of expansion than martensitic or ferritic materials. This results in high stresses at the welded connecting localities between materials of austenitic materials (so-called “white materials”) and those of ferritic or martensitic materials, (so-called “black materials”). Conventionally, so-called black-white-connectors are employed which, in the example according to FIG. 3, are produced from two axially welded together pieces of piping formed of appropriate different materials and which are welded in the unheated region into the connecting ducts 15 (cf FIGS. 1 and 2) from the heated surface to the collector 17 or immediately preceding the collector 17 between the connecting ducts 15 and the collector nipples 16. It is also known to provide such weld connectors on the boiler side between the heating surface outlet and connecting ducts leading to the collector.
Such black-white connectors, because they are subject to increased quality requirements, can only be manufactured and tested at high cost. Moreover, when installing the black-white connectors, several welding seams are needed in the connecting ducts leading to the collector, whereby, in turn, the assembly and testing costs are further increased.